The disclosure relates generally to stress relief in metal components, and more particularly, to a method of converting residual surface stress in an internal opening of an additively manufactured component to a lesser stress or a different type stress.
Conventional manufacture of metal components generally includes milling or cutting away regions from a slab of material before treating and modifying the cut material to yield a part, which may have been simulated using computer models, e.g., in drafting software. Manufactured components which may be formed from metal can include, e.g., airfoil components for installation in a turbomachine such as an aircraft engine or power generation system. Additive manufacturing (AM) includes a wide variety of processes of producing a component through the successive layering of material rather than the removal of material. As such, additive manufacturing can create complex geometries without the use of any sort of tools, molds or fixtures, and with little or no waste material. Instead of machining components from solid billets of material, much of which is cut away and discarded, the only material used in additive manufacturing is what is required to shape the component. Additive manufacturing techniques typically include taking a three-dimensional computer aided design (CAD) file of the component to be formed, electronically slicing the component into layers, e.g., 18-102 micrometers thick, and creating a file with a two-dimensional image of each layer, including vectors, images or coordinates. The file may then be loaded into a preparation software system that interprets the file such that the component can be built by different types of additive manufacturing systems. In 3D printing, rapid prototyping (RP), and direct digital manufacturing (DDM) forms of additive manufacturing, material layers are selectively dispensed, sintered, formed, deposited, etc., to create the component.
In metal powder additive manufacturing techniques, such as direct metal laser melting (DMLM) (also referred to as selective laser melting (SLM)), metal powder layers are sequentially melted together to form the component. More specifically, fine metal powder layers are sequentially melted after being uniformly distributed using an applicator on a metal powder bed. Each applicator includes an applicator element in the form of a lip, brush, blade or roller made of metal, plastic, ceramic, carbon fibers or rubber that spreads the metal powder evenly over the build platform. The metal powder bed can be moved in a vertical axis. The process takes place in a processing chamber having a precisely controlled atmosphere. Once each layer is created, each two dimensional slice of the component geometry can be fused by selectively melting the metal powder. The melting may be performed by a high power melting beam, such as a 100 Watt ytterbium laser, to fully weld (melt) the metal powder to form a solid metal. The melting beam is moved or deflected in the X-Y direction, and has an intensity sufficient to fully weld (melt) the metal powder to form a solid metal. The metal powder bed may be lowered for each subsequent two dimensional layer, and the process repeats until the component is completely formed.
Use of certain materials creates challenges for metal powder additive manufacturing. For example, gamma prime hardened superalloys (e.g., nickel-based) are advantageous materials to produce components using DMLM. “Gamma prime” is a coherently precipitating phase in which there is a close match in matrix/precipitate lattice, resulting in a material having a long-duration stability. After manufacturing, the additively manufactured components using such materials are exposed to a stress relief heat treatment during which the components suffer from macro-cracking caused by high residual surface stresses in the component. This phenomenon may be referred to as strain age cracking (SAC). A potential mechanism to mitigate SAC includes shot peening applied to an external surface of the component to prevent macro-cracking in the external surface, i.e., to introduce compressive stress to compensate for a high residual tensile surface stress. With increased component complexity, especially with intricate internal features like near wall cooling passages, the effectiveness of shot peening is limited because it is only applicable to an external surface of the component, not internal openings. Consequently, certain applications for a component using gamma prime hardened superalloys having complex internal geometries are not feasible.